This invention relates generally to the field of viscosity control technology and is particularly directed to an ultrasonic viscometer device to measure the viscosity and the cure state of a thermally curing polymer composite at high temperature using the reflectivity of ultrasonic shear waves. Such viscosity measurement permits the control and the optimization of the cure cycle to minimize porosity and delaminations in the composite.
Fiber-reinforced composite materials, known as "composites", comprise a base or substrate material, such as an epoxy resin, which is impregnated, for structural strength, with fibers of such materials as carbon, graphite, glass, boron and nylon. Composites typically exhibit extremely high strength-to-weight ratios, and accordingly, their use is becoming increasingly important in commercial and aerospace applications.
Such composite materials, as previously noted, are typically based upon a matrix of thermo-curing polymers or resins, such as epoxies, which are usually cured at high temperatures in an autoclave. Under the effect of increasing temperature, the polymer molecules grow into longer chains and branches. In the case of thermosetting polymers, crosslinks between the chains are also formed. The rate of the reaction is a complex function of the temperature and pressure which depends upon the thickness and geometry of the part being made, on the thermal equilibrium between the part and the mold, on the temperature of the atmosphere around the part and/or the thermal mass of the autoclave. The monitoring of the viscoelastic properties of the resin provide important information on the state of the cure and allow the control of the flow properties. Molding operations may be optimized and the content of voids minimized by a better control of the degree of resin compaction.
Although mechanical measurement of viscosity can be made, this requires the insertion of a probe into the composite part being cured. This operation generally is not feasible during manufacture of a composite part and, particularly, when several determinations of viscosity are necessary from different locations on the part.
Some chemical techniques have been developed to monitor the cure state of a resin but are difficult to implement in a manufacturing environment. Some examples are high-performance liquid chromatography, differential scanning calorimetry, which determines the state of the cure by the heat generation needed to complete the cure, and infrared spectroscopy.
Ultrasonic methods have been used for some time to measure resin viscosity and the cure state thereof. An advantage of ultrasonic wave propagation for measuring the viscosity of a medium, such as a resin during cure thereof, is that it depends directly on the mechanical constants of the medium of propagation. In principle, ultrasonic techniques then provide a way to measure these constants. However, the mechanical constants vary significantly with frequency, sometimes by several orders of magnitude between the range of a few Hertz, where they are a function of the flow properties of the resin, and the MegaHertz range where ultrasonic techniques have been used more often.
Thus, the article, "Dynamic Viscoelastic Properties of Cholesteric Liquid Crystals", by J. F. Dyro, et al, MOL Cryst. & Liq. Cryst., Vol. 29, No. 3-4, 263-84, 1975, discloses use of reflectivity of shear waves to obtain the dynamic shear viscosity of cholesteric liquid crystals from 35.degree. to 55.degree. C. However, this procedure would appear to require a thermal equilibrium bath and could not be performed in an autoclave.
The article, "Ultrasonic Viscometer for the Measurement of Dynamic Shear Viscosity of Liquids", V. N. Bindal, et al, Indian J. Pure Appl. Phys., Vol. 21, No. 3, March, 1983, pp. 176-177, discloses an ultrasonic viscometer for the measurement of dynamic shear viscosity of liquids using a quartz crystal along with a fused quartz delay line. Apparently, the temperatures reached are below 100.degree. C., and the system apparently operates under temperature control, that is, stabilized temperature, conditions. However, the use of such technique in an autoclave with a quickly varying temperature would not be feasible.
The use of ultrasound to monitor the cure of epoxies has been proposed. The technique most commonly used for this purpose has been the measurement of velocity and attenuation of longitudinal and shear waves.
U.S. Pat. No. 4,559,810 to Hinrichs, et al, discloses a method for determining the dynamic viscosity of a specimen of a polymeric resin which is subjected to a time-varying temperature by passing an ultrasonic sensing wave of known amplitude through the specimen, sensing the amplitude of the wave after it has traveled through the specimen, and from the degree of amplitude attenuation, obtaining a value which has a linear relationship to the logarithm of the dynamic viscosity of the resin.